In the very great majority of cases, tread patterns for heavy-vehicle tires are provided with circumferential grooves, whether rectilinear, zigzag or undulating, said grooves possibly being joined by transverse grooves and/or incisions. The circumferential grooves generally comprise wear indicators, small underlayers of vulcanized rubber mix (or rubber) covering the bottom of these grooves over a certain circumferential length, said indicator indicating the minimum tread depth which must legally remain on the tread when in use. The tread patterns for heavy vehicles are regroovable (operation by means of which new grooves can be re-cut), and tires having such tread patterns bear the word “regroovable” or the symbol “U” on their sidewalls. Regrooving makes it possible firstly to extend the grip potential of the heavy-vehicle tire and secondly to increase substantially the possible mileage: from 15 to 30% depending on the case, without adversely affecting the possibility of recapping, which is furthermore an essential characteristic of a heavy-vehicle tire. It should be added that regrooving also makes it possible to save on fuel, since the tire has a lesser rolling resistance, owing to the reduced depth of the grooves in the initial state compared with those of a tire having, when new, an overall tread pattern depth corresponding to the maximum regrooving depth.
As is known per se, the regrooving of a groove can be effected using a heated rounded blade, frequently still manipulated by an operator. Said blade, which is connected to a frame which bears on the tread surface, may be used manually so as to follow the trace of the groove on the surface of the tread fairly faithfully, even in the case of a groove having a non-rectilinear trace. However, this regrooving operation requires a certain number of precautions. The first consists of effecting the regrooving operation when there is about 2 mm of groove depth remaining, said depth being measured between the tread surface and the radially outer surface of the wear indicators placed at the bottom of the groove. This precaution makes it possible to visualize the design of the tread pattern well and thus to reproduce it without major difficulty. Given the depth of tread pattern remaining and the regrooving depth recommended by the tire manufacturer, it is then possible to adjust and control the height of the regrooving blade.
The regrooving depths generally indicated are theoretical depths. Although they are satisfactory in a majority of cases, and make it possible theoretically to control the blade height to maintain approximately a certain thickness of rubber between the bottom of the regrooved groove and the radially upper face of the crown reinforcement, the risks of excessively deep regrooving are not ruled out. Now, excessive regrooving may cause damage causing premature destruction of the tire. It may also compromise the possibility of economic recapping, that is to say, recapping in which only the tread is changed. It may also, in some extreme cases, involve the appearance, at the bottom of the new grooves after regrooving, of the plies of the radially subjacent crown reinforcement, which is not generally accepted by current legislation.
In order to permit regrooving to be carried out which respects precisely the presence of a minimum thickness of rubber, above the radially outer face of the crown reinforcement, set by the tire manufacturer, while making it possible to increase as far as possible the possible mileage of the tire, U.S. Pat. No. 6,003,576 advocates, in a tire, comprising a radial carcass reinforcement radially surmounted by a crown reinforcement formed of at least one ply of reinforcement elements, and a tread provided with grooves which are capable of being regrooved, providing those parts of the tread which are located radially beneath the regroovable grooves with depth indicators, each indicator comprising at least one means indicating the minimum depth to be reached for effective regrooving and the maximum depth which must not be exceeded at any cost.
The depth indicators are preferably in the form of incisions of low width other than zero which are placed at the bottom of the groove either parallel to the direction of said groove or perpendicular to said direction, or both simultaneously, the means indicating the minimum and maximum depths then being the geometric form of the bottom of the depth-indicating incision.
Although they have resulted in great progress in terms of the art and manner of regrooving a tread, the regrooving indicators, despite extensive mechanization and robotisation, do not remove the risk of a cutting blade passing very close to the plies of the crown reinforcement; these indicators do not dispense with the need for a human presence for adjusting the depth. Furthermore, the regrooving is effected radially beneath the original grooves, which were designed as a function of the thickness of a new tread, and not as a function of a tread, the thickness of which has greatly decreased and the optimal tread pattern design of which is not necessarily the design thought up for the normal thickness of the tread.
It has also been proposed (U.S. Pat. No. 2,148,343) to incorporate in the tread of the new tire a plurality of filler arranged within said tread in the longitudinal direction. As soon as the wear of the tread reaches the fillers, the latter are ejected by centrifugal force and new grooves are then formed. This solution results in risks linked to the ejection of pieces of filler of greater or lesser size which when thrown off at high speed represent a potential risk to people or vehicles nearby; furthermore, these pieces pollute the environment.